Gas turbine engines, such as those used to power modern commercial aircraft or in industrial applications, include a compressor for pressurizing a supply of air, a combustor for burning a hydrocarbon fuel in the presence of the pressurized air, and a turbine for extracting energy from the resultant combustion gases. Generally, the compressor, combustor and turbine are disposed about a central engine axis with the compressor disposed axially upstream of the combustor and the turbine disposed axially downstream of the combustor.
In operation of a gas turbine engine, fuel is combusted in the combustor in compressed air from the compressor thereby generating high-temperature combustion exhaust gases, which pass through the turbine. In the turbine, energy is extracted from the combustion exhaust gases to turn the turbine to drive the compressor and also to produce thrust. The turbine includes a plurality of turbine stages, wherein each stage includes of a stator section formed by a row of stationary vanes followed by a rotor section formed by a row of rotating blades. In each turbine stage, the upstream row of stationary vanes directs the combustion exhaust gases against the downstream row of blades. Thus, the blades of the turbine are exposed to the high temperature exhaust gases.
The turbine blades extend outwardly from a blade root attached to a turbine rotor disk to a blade tip at the distal end of the blade. A blade outer air seal extends circumferentially about each turbine rotor section in juxtaposition to the blade tips. Desirably, a tight clearance is maintained between the blade tips and the radially inwardly facing inboard surface of the blade outer air seal so as to minimize passage of the hot gases therebetween. Hot gas flowing between the blade tips and the blade outer air seal bypasses the turbine, thereby reducing turbine efficiency.
In operation of the gas turbine engine, the blade outer air seal is exposed to the hot gases flowing through the turbine. The blade outer air seal is constructed of a plurality of blade outer air seal (BOAS) segments having longitudinal expanse and circumferential expanse and laid end-to-end abutment in a circumferential band about the turbine rotor so as to circumscribe the blade tips. Consequently, it is customary practice to provide for cooling of the BOAS segments, typically using cooler temperature bleed air taken from elsewhere in the engine. Various methods of cooling the BOAS segments are currently in use and typically include impinging the cooler bleed air against the outboard back side of each BOAS segment and commonly also passing cooler bled air through a plurality of air flow passages formed within the body of each BOAS segment. Despite the cooling of the BOAS segments, in the harsh thermal environment to which the blade outer air seal is exposed, particularly in the high pressure turbine section located immediately aft of the combustion chamber, the BOAS segments may over repeated thermal cycles of exposure to the hot gases crack due thermal mechanical fatigue.
Due to the harsh thermal environment, each BOAS segment is made of a high temperature superalloy material, such as single crystal nickel alloys. During engine operation, the radially inboard facing side (ID surface) of each BOAS segment heats up due to exposure to the hot gases passing through the turbine and tries to expand. Since the radially outboard facing side (OD surface) and typically the internal structure of the body of each BOAS segment is exposed to cooling air while the radially inboard facing side of the body of each BOAS segment is exposed directly to the hot gases passing through the turbine section, the BOAS segments are subject to differential thermal expansion. That is, the radially inboard facing side of the body of each BOAS segment, due to exposure to a higher temperature than the radially outboard side, undergoes thermal expansion at a higher rate than the outboard side of the body of the BOAS segment, thereby putting the radially inboard facing side of each BOAS segment into a compressive stress state that at high temperature may exceed the yield point of the material. Upon cooling, residual tensile stress are produced in the material that yielded in the radially inboard side of body at high temperature. After repeated thermal cycling, the residual tensile stresses can form thermal mechanical fatigue cracks on the radially inboard surface of the BOAS segment, which may necessitate premature removal of the engine from service to remove and replace cracked and damaged segments of the blade outer air seal.
Accordingly, there is a need for a blade outer air seal segment that is less prone to cracking due to thermal mechanical fatigue resulting from differential thermal expansion during engine cycling.